Rather over-costed. Paying two more mana over Pit Fighter for a heal feels impractical, this could very easily have been a five or six drop; seven feels way too slow for the body you get. Even in arena it's lackluster.

This card has become an important component of slow Paladin decks, especially post-Naxxramas. A convenient 5/6 minion has been such a taboo in this game. This one costs too much to be convenient, while the 5/6 is difficult for your foe to take out. It's like 2 mana for healing your hero for 6 and 5 mana for a 5/6, too good a bargain to miss if you decide to play an extremely slow deck.

Usually a Paladin deck consists of many minions, 2 swords and few spells, because its drawing is poor and Paladin spells are weak. One Guardian of the Kings is good for such construct strategy. Such Paladin doesn't employ Earthen Ring Farseer because there are too many 3-drops in the deck already, so Guardian of the Kings is in.

Well you're partly right... Its a 5/6 body that doubles as your heal spell. However, this makes room for another 2 mana body in that slot, which in turn makes your deck faster. He's your curve topper whether you play it fast or slow. Also, if you need to gain 6 life to stay alive before turn 7, you're either playing the class wrong, had a horrible draw, or are simply out classed and should scoot anyway.